Skin Tests in the Diagnosis of Allergic Conditions
Lillian P. Kravis MD1
1 Associate Clinical Professor of Pediatrics, University of Pennsylvania Medical School; Associate Physician, Children's Hospital of Philadelphia; Associate Allergist, Children's Hospital of Philadelphia
Allergic disorders may be classified into four main categories of immunologic hypersensitivity reactions: type I, immediate or anaphylactic; type II, cytotoxic; type III, involving immune complexes; and type IV, delayed or cell mediated. Type I reactions are those most commonly encountered in clinical allergy. They may be well defined by skin testing, provided the patients to be tested are carefully selected and the tests are applied properly and interpreted critically.
Major indications for direct skin testing are: (1) to separate atopic from nonatopic patients; (2) to help in identifying specific allergens in atopic persons; and (3) to reassess the allergic profile in patients receiving immunotherapy. Other indications for skin-testing, of more limited applicability, include: (1) screening for hypersensitivity to vaccines; (2) testing for reactivity to drugs (and particularly penicillin); (3) testing for immediate and delayed reactions in such conditions as bronchopulmonary aspergillosis; and (4) in evaluation of patients with reactions to insect stings.
Skin-testing has limited usefulness in early infancy, in chronic urticaria, in the investigation of food allergies, in atopic dermatitis not accompanied by other manifestations of atopy, or in allergic reactions to drugs other than penicillin.
A checklist to be consulted prior to performing or interpreting skin tests should include: (1) assurance of the potency of the extract used in testing; (2) knowledge of the nonspecific irritant properties of any extracts used for testing; (3) history of administration of antihistamine drugs or epinephrine prior to testing; and (4) the correlation of skin test results with clinical history.
The help of a qualified allergist should be sought if: (1) skin test results are at variance with the historical data; or (2) systemic reactions are induced by skin testing; or (3) the patient fails to respond to the therapy dictated by the skin test results.
Alternatives or supplements to skin-testing include RAST testing and measurement of histamine release from sensitized leukocytes. These have rendered passive transfer testing (PK or Prausnitz-Küstner) substantially obsolete.
Direct allergy skin testing remains the most sensitive, most specific, and most reliable method presently available to the physician for investigation of IgE-mediated allergic disorders.
